See Figures 1 and 2
The General Motors HEI system is a pulse-triggered, transistor controlled, inductive discharge ignition system. Except on 1975-77 inline 6-cylinder models, the entire HEI system is contained within the distributor cap. 1975-77 inline 6-cylinder engines have an external coil. Otherwise, the systems are the same.
The distributor, in addition to housing the mechanical and vacuum advance mechanisms, contains the ignition coil (except on some inline 6-cylinder engines), the electronic control module, and the magnetic triggering device. The magnetic pick-up assembly contains a permanent magnet, a pole piece with internal teeth, and a pick-up coil (not be be confused with the ignition coil).
In the HEI system, as in other electronic ignition systems, the breaker points have been replaced with an electronic switch, a transistor, which is located within the control module. This switching transistor performs the same function the points did in a conventional ignition system; it simply turns coil primary current on and off at the correct time. Essentially then, electronic and conventional ignition systems operate on the same principle.
The module which houses the switching transistor is controlled (turned on and off) by a magnetically generated impulse induced in the pick-up coil. When the teeth of the rotating timer align with the teeth of the pole piece, the induced voltage in the pick-up coil signals the electronic module to open the coil primary circuit. The primary current then decreases, and a high voltage is induced in the ignition coil secondary windings which is then directed through the rotor and spark plug wires to fire the spark plugs.
In essence then, the pick-up coil module system simply replaces the conventional breaker points and condenser. The condenser found within the distributor is for radio suppression purposes only and has nothing to do with the ignition process. The module automatically controls the dwell period, increasing it with increasing engine speed. Since dwell is automatically controlled, it cannot be adjusted. The module itself is non-adjustable and non-repairable and must be replaced if found defective.
HEI SYSTEM PRECAUTIONS
Before going on to troubleshooting, it might be a good idea to take note of the following precautions:Timing Light Use
Inductive pick-up timing lights are the best kind to use with HEI. Timing lights which connect between the spark plug wire occasionally (not always) give false readings.Spark Plug Wires
The plug wires used with HEI systems are of a different construction than conventional wires. When replacing them, make sure you get the correct wires, since conventional wires won't carry the voltage. Also handle them carefully to avoid cracking or splitting them and never pierce them.Tachometer Use
Not all tachometers will operate or indicate correctly when used on a HEI system. While some tachometers may give a reading, this does not necessarily mean the reading is correct. In addition, some tachometers hook up differently from others. If you can't figure out whether or not your tachometer will work on your truck, check with the tachometer manufacturer. Dwell readings have no significance at all.HEI System Testers
Instruments designed specifically for testing HEI systems are available from several tool manufacturers. Some of these will even test the module itself. However, the test given in the following section will require only an ohmmeter and a voltmeter.